:]28 W, M. Bale: 



The salient character is in the origin of the proboscis, which is 

 derived from the tentacles. Tliese are united in the proximal 

 portions, so as to form a calyx, lined by an endodermal layer, which 

 extends to its margin, at Avhich part the tentacles become free. 

 From the edge of the calyx springs the proboscis, Avliich is normally 

 dome-shaped, with a small central aperture, but which, when tlie 

 liydranths are fully expanded, stands erect and open, like the 

 corona of a Narcissus. This corona iis seen in optical section to 

 <_onsist of a mesosarc branching ott' from the united mesosarcs of the 

 tentacular calyx, with, outside it, a thin pellicle of ectoderm, and 

 inside a thick layer, presumably endodermal, whicli continues 

 downward uninterruptedly into the layer of similar tissue which 

 lines the calyx. As usual in the order, this tissue is denser and 

 closer than the endoderm in the lower part of the body; it forms a 

 layer with a well-delined inner boundary, and is uniform in thick- 

 ness for most of its extent, but becomes thicker opposite the extreme 

 bases of the tentacles, below which it gradually passes over into the 

 ordinary endoderm of the body cavity. 



The difference between this type of hydranth and that of a typical 

 Campanularian, such as Ohella, is very marked. In Obelia it is 

 true the basal portions of the tentacles are adherent, forming a 

 calyx, but this is composed of the tentacles alone, which are borne 

 on the hydranth quite apart from the large mobile proboscis, which 

 they surround at some distance. In Silicularia, on the other hand, 

 we have a hypostome of which the lower half is composed of the 

 tentacular calyx, with its cellular lining, while the upper half 

 (proboscis), is a free extension of this same calyx. The structure 

 may probably be explained on tlie assumption that the (udinary 

 proboscis has been modified by its proximal portion becoming com- 

 pletely adherent to the tentacular calyx (forming its lining), while 

 tliQ upper part remains free and retains its mobility. 



The proboscis, or free portion of the hypostome, is encircled by 

 ti band of irregular rounded or convoluted masses of a special 

 granular tissue, which project from its outer surface with a thick- 

 ness about equal to that of the proboscis-wall itself. Their function 

 is not obvious. 



The hydranth-body is roughly globular, Avitli a distinct salient 

 angle Arliich fits into a small sinuation in the hydrotheca. Its 

 diameter, from the base to the root of the tentacles, averages al)out 

 .22 nun., and the distance thence to the edge of the pioboscis is 

 iibout .15 mm. The proboscis, when erect, is about .20 across. 



